Shock absorbers for automobiles usually include a dashpot in which a piston reciprocates within a fluid containing cylinder, the piston rod projecting from the cylinder through a guide aperture in the end wall for attachment to a vehicle part. Due to accidents, such as collisions, the piston rod not infrequently becomes bent, thereby destroying the necessary concentricity between the piston rod and cylinder. Such eccentricity of the piston rod may be slight and virtually impossible to determine by the naked eye, but even though it is slight, the action of the shock absorber is impaired and its effectiveness diminished if not permanently damaged. Thus, it has been extremely difficult for a service man to ascertain the efficient operability of the shock absorber.